The present invention relates to an electronically commutatable motor, whose field windings are controllable by an electronic control unit via semiconductor output stages using PWM control signals, a nominal setpoint speed being selectable for the control unit and the actual speed being comparable with the setpoint, and when the actual speed deviates from the setpoint by a selected or selectable speed difference, the control unit and/or the semiconductor output stages are capable of being switched off.
An electronically commutatable motor is discussed in U.S. Pat. No. 5,744,921. This motor is operable at different nominal operating speeds, the setpoint applied determining the PWM control signals and the associated nominal operating speed. Each operating state may be monitored by comparison and deviation between the actual speed and an associated setpoint speed. The motor may be switched off when the speeds differ by a selected speed deviation. In this motor, the control unit may be capable of modifying the PWM control signals so that the motor runs in the correct direction of rotation for the operating state as a function of the rest position of the rotor.
The motor may be switched off in the event of a speed deviation from the setpoint by a selected value, as discussed in Japanese Patent No. 10 322 894, where the motor may be switched off by switching off the supply voltage, as discussed in German Published Patent Application No. 43 30 823.
These types of electronically commutatable motors may be used for various applications. In this regard, it may happen that even the acceleration of the motor to a desired nominal operating speed, which is selectable or selected via a setpoint, does not occur in the correct way. The motor may be capable of stalling in the rest position or prevented from accelerating correctly by an excessive load or other impediments. Mechanical defects may also cause such a phenomenon. The motor may also block in any phase of the acceleration. The motor and/or its electronic components may be overloaded and even damaged.
It is the object of the present invention to monitor and switch off an electronically commutatable motor of the type initially described, during each phase of the acceleration in such a way that an overload or damage to the motor is prevented as early as in this acceleration phase, rather than in the desired operating state.
In an exemplary embodiment of the present invention, with the selection of the setpoint and/or switching on the control unit and/or the semiconductor output stages as a function of the selected setpoint, the control unit determines a corresponding acceleration time in which the PWM control signals, which gradually increase the speed, increase to a final value associated with the selected setpoint. In the control unit a setpoint speed is associated with each control value of the PWM control signals. In each control phase of the acceleration time, the actual speed is measured and compared with the setpoint speed associated with the corresponding PWM control signal. Switch-off occurs as early as during acceleration if in a control phase the measured actual speed deviates by the selected speed difference from the setpoint speed associated with the corresponding PWM control signal.
The acceleration phase is divided into time segments by the gradually increasing PWM control signals; in each case the coincidence of the actual speed with the setpoint (i.e., the PWM control signal being applied) also corresponds to the motor characteristic and indicates undistrubed acceleration of the motor. If a deviation from a permissible speed deviation occurs in a time segment of the acceleration phase, a mechanical or electric malfunction may be present, which may justify the motor being switched off in order to protect the motor or its components from overload and damage.
In this manner, the entire acceleration phase may be monitored for correct start and acceleration from the time the motor is switched on.
The PWM control signals which gradually increase the speed may continuously increase in pulse amplitude, pulse width, and/or commutating frequency.
The actual speed may be compared to the setpoint speed continuously or repeatedly at time intervals during the acceleration.
The simple protective circuit may be obtained by switching the control unit and/or the semiconductor output stages on and off by switching the supply voltage on and off.
For different operating modes, the design may advantageously be such that the acceleration time depends on the magnitude of the selectable setpoint and/or on the magnitude of the supply voltage.
The variation of the permissible speed deviation may be designed so that the selected speed deviation depends on the selected setpoint and/or on the magnitude of the supply voltage.
If necessary, the deviation from the selected speed deviation may be visually displayed and/or acoustically indicated.
The comparison device required to compare actual speed and setpoint speed may be integrated into the control unit.
For the duration of the operation of the motor with the operating speed defined by the selected setpoint, the comparison between actual and setpoint speed may be continued to obtain overload protection also in this phase of the steady-state operation and be capable of switching off if necessary.